An international team of researchers is working to develop a vaccine against botulism, a deadly nerve toxin that could be used as a biological weapon.

The vaccine is carried in tiny glass spheres, which allow the 'dormant' jab to come to life once it's injected into the body.

UK company Cambridge Biostability will work on the vaccine with DynPort Vaccine Company of the US.

Botulism is caused by botulinum neurotoxin, one of the most poisonous naturally occurring substances. Eating food contaminated with the toxin can lead to paralysis and death.

The toxin is also one of the agents experts believe is most likely to be used in bioterrorism.

But developing a single vaccine against botulism has been difficult. This is because the bacterium Clostridium botulinum and other closely related bacteria produce several different types, or serotypes, of toxin.

"Botulism is caused by seven slightly different poisons produced by six different bugs. In the event of an attack, a person would need to be given multiple vaccine injections to cover all the bugs and three shots are required of each vaccine," says Bruce Roser, chief scientific adviser for Cambridge Biostability.

To overcome the obstacle, the company will use technology it developed to encapsulate each poison in its own microsphere, spray-dried sugar syrup that hardens like glass.

The microspheres can then be mixed and kept at room temperature without harming the vaccine.

"This means that for the first time it is possible to create a single multivalent vaccine which can be stored safely without refrigeration," adds Roser.

Tweaking the toxins

DynPort Vaccine Company will be involved in producing the seven botulin poisons for the vaccine. By altering the genes that control how they are made, the company hopes to produce weakened versions of the toxins, called toxoids.

While these toxoids will raise an immune response, they will not cause full-blown botulism, the researchers say.

Symptoms of botulism start after six hours to two weeks of being exposed to the toxin, either by eating contaminated food or by inhaling the toxin in powder form.

Symptoms include double or blurred vision, drooping eyelids, slurred speech and muscle weakness that descends from the shoulders to the legs.

If the toxin is converted into a fine powder and released into the air, only a few grams could affect millions of people if inhaled.